I added some minor improvements of my own, mainly:
Different colors for the bullets, player, and invader
"Start Game" / "Play Again" buttons, with "Game Over" / "Player Wins" message
Bullets can't destroy each other
Limit player's firing rate (it was basically a machine gun as implemented)
Variation in invaders' color and design
"Classic" Space Invaders color scheme and old school font
Check it out.
Source code can be found at my Github repo.